Organic electronic devices, such as organic semiconductors, can be used to fabricate simple electronic components, e.g. resistors, diodes, field effect transistors, and also optoelectronic components like organic light emitting devices (e.g. organic light emitting diodes (OLED)), and many others. The industrial and economical significance of the organic semiconductors and their devices is reflected in the increased number of devices using organic semiconducting active layers and the increasing industry focus on the subject.
OLEDs are based on the principle of electroluminescence in which electron-hole pairs, so-called excitons, recombine under the emission of light. To this end the OLED is constructed in the form of a sandwich structure wherein at least one organic film is arranged as active material between two electrodes, positive and negative charge carriers are injected into the organic material and a charge transport takes place from holes or electrons to a recombination zone (light emitting layer) in the organic layer where a recombination of the charge carrier to singlet and/or triplet excitons occurs under the emission of light. The subsequent radiant recombination of excitons causes the emission of the visible useful light emitted by the light-emitting diode. In order that this light can leave the component at least one of the electrodes must be transparent. Typically, a transparent electrode consists of conductive oxides designated as TCOs (transparent conductive oxides), or a very thin metal electrode; however other materials can be used. The starting point in the manufacture of an OLED is a substrate on which the individual layers of the OLED are applied. If the electrode nearest to the substrate is transparent the component is designated as a “bottom-emitting OLED” and if the other electrode is designed to be transparent the component is designated as a “top-emitting OLED”. The layers of the OLEDs can comprise small molecules, polymers, or be hybrid.
Several operational parameters of the OLED are being constantly improved to enhance the overall power efficiency. One important parameter is the operation voltage which can be tuned by improving the transport of charge carriers and/or reducing energy barriers such as the injection barriers from the electrodes, another important figure is the quantum efficiency, and also very relevant is the lifetime of the device. Other organic devices, such as organic solar cells also require improving in efficiency, which nowadays, are at best at about 9%.
Like an OLED, an organic solar cell has a stack of organic layers between two electrodes. In a solar cell, there must be at least one organic layer responsible for the absorption of light and a interface which separates the excitons created by the absorption (photo-active). The interface can be a bi-layer heterojunction, a bulk-heterojunction, or can comprise more layers, e.g., in a step wise interface. Also sensitizing layers and others can be provided. For increased efficiency, a good charge carrier transport is required, in some device structures the transport regions must not absorb light, therefore transport layers and photo-active layers may comprise different materials. Also charge carrier and/or exciton blocking layers may be employed. Highest efficiency solar-cells are, nowadays, multi-layer solar cells, some device structures are stacked (multi-junction solar cells) and connected by a connecting unit (also called recombination layer); nevertheless, single junction solar cells could have a high performance if the right materials were found. Examples of devices are given in US2009217980, or in US2009235971.
Differently than OLEDs and organic solar cells, transistors do not require doping of the entire semiconducting (channel) layer, because the concentration of available charge carriers is determined by an electric field supplied by a third electrode (gate electrode). However, convention organic thin film transistors (OTFTs) require very high voltages to operate. There is a need to lower this operating voltage; such an optimization can be done, e.g. with appropriate injection layers.
Organic transistors are also called organic field-effect transistors. It is anticipated that a large number of OTFTs can be used for example in inexpensive integrated circuits for non-contact identification tags (RFID) but also for screen control. In order to achieve inexpensive applications, generally thin-layer processes are required to manufacture the transistors. In recent years performance features have been improved to such an extent that the commercialization of organic transistors is foreseeable. For example, in OTFTs high field-effect mobilities of up to 5.5 cm2/Vs for holes on the basis of pentacene (Lee et al., Appl. Lett. 88, 162109 (2006)) have been reported. A typical organic field-effect transistor comprises an active layer of organic semiconducting material (semiconducting layer) material which during the operation forms an electrical conduction channel, a drain and a source electrodes which exchange electrical charges with the semiconducting layer, and a gate electrode which is electrically isolated from the semiconducting layer by an dielectric layer.
There is a clear need to improve charge carrier injection and/or conductivity in organic electronic devices. Reducing or eliminating the barrier for charge injection between the electrode and the electron transport material (ETM) contributes strongly to enhancement of the device efficiency. Nowadays, there are two main approaches to reduce voltage and enhance efficiencies of organic electronic devices: improvement of the charge carrier injection and improvement of the conductivity of the transport layers. Both approaches can be used in combination.
For instance, U.S. Pat. No. 7,074,500 discloses a component structure for an OLED which leads to a greatly improved charge carrier injection from the electrodes into the organic layers. This effect is based on considerable band bending of the energy levels in the organic layer at the interface to the electrodes, as a result of which injection of charge carriers on the basis of a tunnel mechanism is possible. The high conductivity of the doped layers also prevents the voltage drop which occurs there during operation of the OLED. The injection barriers which may occur in OLEDs between the electrodes and the charge carrier transport layers are one of the main causes for an increase in the operating voltage compared to the thermodynamically justified minimum operating voltages. For this reason, many attempts have been made to reduce the injection barriers, for example by using cathode materials with a low work function, for example metals such as calcium or barium. However, these materials are highly reactive, difficult to process and are only suitable to a limited extent as electrode materials. Moreover, any reduction in operating voltage brought about by using such cathodes is only partial.
Metals having low work function, in particular alkali metals such as Li and Cs, are often used either as the cathode material or the injection layer to promote electron injection. They have also widely been used as dopants in order to increase the conductivity of the ETM, U.S. Pat. Nos. 6,013,384, 6,589,673. Metals such as Li or Cs provide a high conductivity in matrixes which are difficult to dope otherwise (e.g. BPhen, Alq3).
However, the use of low work function metals has several disadvantages. It is well known that the metals can easily diffuse through the semiconductor, eventually arriving at the optically active layer quenching the excitons, thereby lowering the efficiency of the device and the lifetime. Another disadvantage is their high susceptibility to oxidation upon exposure to air. Therefore, devices using such metals as dopants, injection or cathode material require rigorous exclusion of air during production and rigorous encapsulation afterwards. Another well known disadvantage is that higher molar doping concentration of the dopant exceeding 10 mol. % may increase the undesired absorption of light in the transport layers. Yet another problem is high volatility of many simple redox dopants like Cs, leading to cross-contamination in the device assembling process making their use in device fabrication tools less attractive.
Another approach to replace metals as n-dopants and/or injection materials for ETM is the use of compounds with strong donor properties, such as tetrakis(1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidinato)ditungsten (II) (W2(hpp)4) or Co(Cp*)2 (US2009/0212280, WO2003/088271) which have similar or slightly less doping/injecting ability in comparison with alkaline earth metals. Due to their still high enough electron donating capability, they are also undergoing rapid decay upon exposure to air, making their handling in device production difficult.
It is also known to mix metal organic complexes such as lithium quinolate (LiQ) into an electron transport layer to improve the device, however the exact mechanism of improvement is not well known. Investigations have shown that the use of LiQ still leads to OLEDs with high operating voltage.
Therefore, it is very desirable to provide materials which possess high doping/charge injection capability allowing for highly efficient organic electronic devices substantially preserving the long-term stability of the device and which are infinitely stable in air.
It is therefore an objective of the present invention to provide an organic electronic device, which overcomes state of the art limitations mentioned above and have improved performance compared to electronic devices of the prior art. It is especially an object, to provide an organic electronic device having reduced operating voltage and longer life time reflecting into higher power efficiency.